The invention relates to improvements in apparatus (also called steam boxes) which can be used in paper making and like machines to treat running webs of moist fibrous material. More particularly, the invention relates to improvements in apparatus of the type wherein a housing extends transversely of the path of a running web of moist fibrous material and the housing has one or more plenum chambers adjacent an apertured wall which is placed next to one side of the running web and defines therewith a treating zone receiving steam by way of apertures in the wall and serving to ensure that the steam subjects successive increments of the running web to a predictable treatment.
It is already known to subdivide the interior of the housing of a steam box into a plurality of plenum chambers and to install in the plenum chambers valves which admit steam for distribution within the respective plenum chambers prior to penetration through the apertured wall and into the treating zone. Reference may be had to U.S. Pat. No. 4,422,575 granted Dec. 27, 1983 to Dove for "Steam distributor with plug valve". The plenum chamber of the patented apparatus is flanked by cylindrical steam supplying channels one of which is located upstream and the other of which is located downstream of the plenum chamber in the direction of advancement of the web. The apertured wall has a concave side which faces the path for the web and the wall is provided with a number of apertures for admission of steam into a treating zone between the concave side of the wall and the running web. Each valve of the patented apparatus has a relatively small number of ports, and each port is a large-diameter hole serving for admission of large quantities of steam into the respective plenum chamber. The ports are oriented downwardly, namely toward the apertured wall.
Dove further discloses a flat wall which is formed with several rows of apertures for admission of jets of steam into the treating zone at one side of the running web. A steam supplying conduit extends longitudinally of the plenum chamber above the apertured wall and each valve is connected with an upright pipe having an end close to the apertured wall.
The purpose of the patented apparatus is to raise the temperature of the running web as a result of condensation of steam which is admitted into the treating zone at one side of the web. Such heating of the web facilitates a reduction of its moisture content and thus contributes to simplification and acceleration of the web drying operation. The valves ensure that the treated web can exhibit a desirable moisture profile transversely of the direction of its travel along the apertured wall of the steam box. As a rule, the web is advanced at an elevated or very high speed so that the intervals of time for adequate treatment of successive increments of the running web are very short. Non-uniform treating of successive increments of the web results in considerable fluctuations of temperature and entails non-uniform drying of the paper strip which is obtained from the web. It has been found that heretofore known apparatus of the type known as steam boxes cannot ensure predictable heating of all regions of a running web, especially if the web is transported at an elevated speed.